Learn these 4 concepts and you're on the way to becoming a geologist (and geological detective!)

1) Original Horizontality: sediments are usually deposited in horizontal layers. Used to find folded and tilted rock layers.

2) Lateral Continuity: rock layers usually are deposited over a wide area. Used to look for breaks in continuity that suggest either erosion has occurred or a fault has occurred (see break -- fault -- in reddish rock layer in right image).

3) Superposition: younger rock layers are deposited on top of older rock layers. Used to derive the relative ages of rocks.

4) Law of Crosscutting Relationships: rock that has forced its way up (intruded) into existing rock (e.g., the red rock in left image above) is younger than the existing rock (really just a corollary of superposition).